It is getting to be that time of year, so the LBS will start stocking new lights and discounting last years models. I got both our lights from Healthy Habits. Mine is 250 lumen and the wife has a 300 lumen light. They are pretty bright. Do not look directly into a bike light! You will see black spots for at least 20 seconds. They are that bright.

If I'm riding by myself at night then I will mount the 250 to my helmet and the 300 on the handlebar. If you can only have one light then it should be mounted to your helmet since you need to see where you are looking, not where your handlebars are looking lol. But it's totally not necessary to have both. Beware that higher lumen lights will have a larger power source and the whole thing will be heavier so you probably don't want all that weight on your head of all places. That's why I like the 250. It's small, the battery is inside it, and it's bright enough for me to get my MTB jollies in the dark.

So anyways, minimum I would say is 250 lumen. You can go to 1000 lumen and probably beyond but at a certain price point I stop looking because it just starts to get ridiculous, and you might have a lawsuit on your hands if you blind the crap out of someone with your million dollar plasma powered rival of the sun bike light system.